An example of a device for monitoring a laser cutting process is disclosed in DE 10 2005 024 085 A1. To monitor a laser machining process, the device described in that reference has, inter alia, a camera and an imaging apparatus, which images the region from the zone of interaction or the region of interaction between laser beam and workpiece to be observed on the camera. The output signals from the camera are fed to an evaluation circuit, which processes both the signals from the camera and the signals from a radiation-sensitive receiver, and are used to characterize the course of the laser machining operation. Here, the radiation-sensitive receiver and the camera can cover different spectral ranges. No further information is given about the features used for characterization or the specific evaluation thereof.
The reference WO 91/04828 likewise discloses a monitoring apparatus, in which a camera arranged on the laser machining head, coaxially with the optical axis of a laser beam guided in the direction of the workpiece, is used for focal position determination during a laser cutting process. Here, the camera detects a zone of interaction between laser beam and workpiece and, by using the width of the zone of interaction, conclusions are drawn about the focal position or about the distance between laser machining head and workpiece.
The reference DE 10 2008 051 459 A1 discloses a further such monitoring apparatus which, in particular, is used for edge detection during the layer by layer machining of bodies by means of laser radiation. The device comprises an imaging detector for forwarding a digital image, converted into gray stages or true-color/color-coded, to a data processing system.
The reference DE 43 36 136 C2 describes a method for laser machining in which the laser light reflected at the workpiece, together with generated secondary light, passes back to a laser oscillator and there, with the aid of a mirror, some of the laser light and of the secondary light is separated off. The secondary light is captured by an optical sensor, separately from the laser light component, and a control signal for controlling the laser machining is derived from the remaining secondary light component. In one exemplary embodiment, the surface of the workpiece is irradiated and the reflected radiation passing through a nozzle opening is detected in order to determine a cutting path or cutting point during the laser cutting. The position of the cutting point is compared with the center of the nozzle in order to control the laser cutting process such that the position of the cutting point coincides with the center of the nozzle. In addition, by using the observation of the nozzle opening, the deformation or a blockage of the nozzle opening is determined. Furthermore, in relation to nozzle eccentricity, the reference EP 1 728 581 A1 discloses a device and a method for aligning a laser beam with the nozzle center, in which an image of an illuminated nozzle and a focused laser beam respectively are captured and related to each other via an image evaluation unit.
Furthermore, in order to detect material burn-up, an example of monitoring the shaping of the cutting front (“red heat region”) is known from the reference JP 07116885. If the latter expands, the system changes over to using an inert gas as cutting gas instead of oxygen. For the same purpose or to distinguish between correct and incorrect machining, the reference JP 11320149 discloses an assessment by using a comparison between captured optical signals. In the reference DE 101 29 751, in order to detect material burn-up, the temperature of the workpiece in the vicinity of the cut is monitored by using infrared temperature measuring apparatus and is compared with a temperature limit.
On the basis of the foregoing references, cited by way of example, it becomes clear that the capturing and evaluation of a multiplicity of characteristics determining the quality of the laser cutting process on the basis of devices and methods based on various capturing and evaluation principles is very complex. This relates both to the structure of the device itself and to the signal processing.
However, via the devices described above and the associated methods for evaluating the process images captured, no complete image which would be suitable for characterizing the entire laser cutting process results. In particular, the cut quality itself is inadequately reproduced and control is not carried out comprehensively over the entire process course. In this connection, the entire process course is understood not only as the cutting operation as such but it is also possible for the process course to comprise both the piercing operation and also a plurality of successive laser cuts within the context of a process sequence.